Methods and apparatus for providing quality-of-service guarantees in computer networks

ABSTRACT

An arbitration mechanism provides quality of service guarantees for time-sensitive signals sharing a local area computer network with non-time-sensitive traffic. Device adapters are placed at all access points to an Ethernet network. The device adapters limit admission rates and control the timing of all packets entering the network. By doing so, collisions are eliminated for timesensitive traffic, thereby guaranteeing timely delivery. A common time reference is established for the device adapters. The time reference includes a frame with a plurality of phases. Each of the phases is assigned to a device adapter. Each device adapter is allowed to transmit packets of data onto the network only during the phase assigned thereto. The length of the phases may be modified in accordance with the number of packets to be transmitted by a particular device adapter. A master device adapter may be appointed to synchronize each of the device adapters.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of U.S. Patentapplication Ser. No. 10/688,787, filed Oct. 17, 2003 now abandoned,which is a continuation-in-part application of U.S. application Ser. No.09/764,799, filed Jan. 17, 2001now U.S. Pat. No. 6,661,804, which is acontinuation of Ser. No. 09/224,577, filed Dec. 31, 1998,U.S. Pat. No.6,246,702, which is a continuation-in-part of Ser. No. 09/136,706, filedAug. 19, 1998,U.S. Pat. No. 6,215,797, which are hereby incorporated byreference in their entirety.

FIELD OF THE INVENTION

The present invention is related to computer networks and, moreparticularly, to network apparatus and associated methods that allowsreal-time traffic such as telephone and video to share a computernetwork with non-real-time traffic. The methods and apparatus of thepresent 10 invention provide quality-of-service latency and bandwidthguarantees for time-sensitive signals sharing, for example, an Ethernetnetwork with non-time sensitive signals.

BACKGROUND OF THE INVENTION

Computer telephony, that is, the delivery of telephone calls overcomputer networks, has recently become a focus of attention due to thepotential cost savings of sharing these modern high-bandwidth facilitiesfor multiple uses. Because computer networks packetize signals and thenmix such packetized signals (or more simply, packets) from many sourcesover a single link, networks can make more efficient use ofcommunications resources than conventional circuits witched telephonesystems. Furthermore, computer networks leverage the mass-productioncost savings and technological advances of commodity products. Thissharing of computer communications for non-computer signals thereforehas the potential to greatly lower the cost of communications when usedwith telephone signals.

Computer network traffic from telephone, video, and other time-sensitivesources are generally referred to as real-time traffic because suchtraffic must arrive at a destination within a specified deadline.Real-time traffic generated from audio or video sources is usuallygenerated in equally spaced time intervals. This type of periodicreal-time traffic is referred to as isochrotrnous traffic.

When isochronous traffic is digitized and combined with thesophisticated computer-processing compression techniques, the result isa significant reduction in bandwidth requirements. This use of computertechnology to send telephone and video signals thereby results in evenfurther cost savings.

However, conventional computer networks are not designed to handlereal-time traffic, Collisions and congestion can induce delays andretransmissions, and can cause real-time traffic, such as video, audio,telemetry, and control signals, to arrive late at a destination, therebymissing a deadline. Furthermore, such collision-induced delays arestochastic by nature and therefore unpredictable. Isochronous trafficsources become bursty after traveling through such networks. As aresult, the quality of telephone calls placed over the Internet andcomputer networks in general is very poor at present.

Ethernet computer networks, in particular, use a form of media accesscontrol known as Carrier Sense Multiple Access with Collision Detect(CSMA/CD), also sometimes known as Aloha. This protocol is described indetail by the IEEE Standard 802.3. It provides a very simple andeffective mechanism for allowing multiple packet sources to share asingle broadcast computer network medium. To transmit a new packet, atransmitter need only listen to the network to sense that no packet iscurrently being transmitted. As a transmitted packet is broadcast to allreceivers on the local network, listening to the network for activity istrivial. If a transmitter wishing to send a packet senses that a packetis currently being transmitted, then the transmitter defers transmissionuntil it senses that the network is inactive. Collisions naturally ariseas part of this mechanism. The most common scenario leading to acollision is where two or more stations, which are deferring their ownrespective transmissions during the transmission of another packet,sense a lack of activity at nearly the same time. The protocol detectscollisions, and then aborts and reschedules transmission of all packetsfor a random time later. This protocol, while simple and effective forcomputer traffic, introduces collisions and delays as part of itsnatural operation. In fact, overloading such a network causes the entirenetwork to become unusable, resulting in a significant reduction inthroughput.

Ethernet is now ubiquitous throughout the Internet within local-areacomputer networks, or intranets. The use of variable packet sizes andCarrier Sense Multiple Access with Collision Detect for link access andcontrol creates an even less predictable and less controllableenvironment for guaranteeing quality of service. This is of particularconcern for wide-area real time traffic that must traverse a pluralityof Ethernet networks in order to reach a final destination.

DESCRIPTION OF RELEVANT PRIOR ART

A conventional Ethernet network 1 is shown in FIG. 1 a. ConventionalEthernet devices 100, such as personal computers and printers, generatenon-real-time traffic and are referred to herein as Non-Real-TimeDevices (NRTDs). The NRTDs 100 have a standard Ethernet interface IO andattach to the conventional Ethernet network 1 through Network InterfacePoints 2. The Network Interface Points 2 could represent a 10Base-Tport, a 100Base-TX port, a 10Base-(ThinLAN) port, for example. TheNetwork Interface Points 2 may be interconnected by Repeaters orEthernet Hubs 3.

In conventional Ethernet networks, the attached devices 100 are calledstations. When a station transmits a packet on the network, the signalis broadcast throughout the network. For a transmission to besuccessfully received by another station, there must be no othersimultaneous transmissions. Thus, an arbitration mechanism to share thenetwork is required. Ethernet networks use an arbitration mechanismknown Carrier Sense Multiple Access with Collision Detect (CSMA/CD).

FIG. 1 b provides an example that illustrates how the CSMA/CD protocolworks. A time line of events is illustrated, representing the actions offive stations, labeled Station A, Station B, Station C, Station D, andStation E. These five stations could represent the five NRTDs in FIG. 1a, for example. In this example, Station A transmits a packet 10 on thenetwork after sensing that the network is idle. During the transmissionof this packet 10, Station B generates a packet 12 to transmit on thenetwork, but defers the transmission (indicated by numeral 11) becauseStation B senses activity on the network, due to the transmission 10from Station A. As soon as Station B senses that the network is idle,Station B waits an additional amount of time, known as the Inter-PacketGap (IPG)19, prior to transmitting a packet onto the network. In 10Mbit/sec Ethernet networks, for example, the IPG is defined to be 9.6microseconds, or 96 bit times. This constraint results in a minimum timespacing between packets. After Station B waits for an additional LPGseconds, it transmits the queued packet 12. Accordingly, by sensing thenetwork for activity, collisions can be avoided. Collisions, which occurwhen two or more stations transmit simultaneously on the network, arestill possible, however, due to non-zero latency of detecting the stateof the network and non-zero propagation delay of signals between thestations. As shown in FIG. 1 b, for example, after Station B finishestransmitting a packet 12, the network becomes idle. Sometime later,Station C transmits a packet 13 on the network after sensing that thenetwork is idle. During this transmission from Station C, both StationsD and E each happen to generate a packet for transmission onto thenetwork. As activity is detected on the network, due to the transmission13 from Station C, Stations D and E defer their respective transmissions(indicated by numerals 14 and 15) until the network is sensed idle.Stations D and E will sense that the network is idle at nearly the sametime and will each wait an additional IPG 19 before transmitting theirrespective packets. Station D and Station E will then start transmittingpackets on the network at nearly the same time, and a collision 16 thenoccurs between Station D and station E. The second station to starttransmitting during the collision, say Station B, may or may not be ableto detect the berg of the transmission from the first station thatstarts transmitting, say Station D. In the latter case, Station B doesnot know that a collision will occur when beginning transmission. In theformer case, Station B is still allowed to start transmitting thepacket, even though Station E “knows” that transmission will cause acollision, as long as no activity is detected during the first ⅔ of theIPG. This provision provides a degree of fairness in preventing certainstations from monopolizing the network, due to timing differences acrossstations or location-dependent factors. During the initial part of thetransmissions from Stations D and B, both stations sense that acollision 16 occurs, continue to transmit for 32 bit times, and thenabort the transmission. The process of prolonging the collision for 32bit times is called “jamming” and serves the purpose of ensuring thatall stations involved in a collision will detect that a collision has infact occurred. By aborting transmission after the “jamming” process, thenetwork becomes idle sooner than otherwise. After a station involved ina collision aborts transmission, such a station waits a random amount oftime before attempting to transmit again. If the stations involved inthe collision wait for different amounts of time, another collision isavoided.

The process of waiting a random amount of time until attemptingtransmission again, after aborting a transmission due to a collision, iscalled “backing off” The CSMA/CD protocol uses a backing-off mechanismknown as binary exponential back of, which is now described. A slot timeT is defined to be 512 bit times. For example, in 10 Mbit/see Ethernetnetworks, slot time T is approximately 50 microseconds. After a stationexperiences k collisions for a given packet it is attempting to transmitthe station waits for a time iT before attempting to transmit again,where i is a random integer in the range 0<i<2^(m) and m=min(k, 10).Notice that for a packet experiencing multiple collisions, the averagewaiting time after each collision doubles until 10 collisions haveoccurred. After 16 collisions, the station will discard the packet. Sucha process provides a mechanism for dynamic load adjustment-manycollisions imply a congested network, so the rate of retransmissions isreduced to decrease the probability of further collisions.

After backing off, a station again senses the network for activity,deferring if necessary before transmitting again. For example, as shownin FIG. 1 b, while Station D is backing off (indicated by numeral 17),Station F generates and transmits a packet 18 after detecting that thenetwork is idle. When through backing off, Station D senses activity onthe network, due to the transmission 18 from Station F, and thus defers21 retransmission of the packet. After sensing that the network is idle,Station D then retransmits 22 the original packet that collided earlier,after waiting for IPG 19 seconds. In this example, Station E backs off(indicated by numeral 20) for a longer amount of time, and when StationE is through backing off, Station E senses that the network is idle.Station E then retransmits 23 the packet that collided earlier. Finally,in this example, Station C generates another packet 25 during theretransmission 23 of the packet from Station E, and Station C defers 23transmission until IPG 19 after Station E completes retransmission.

As discussed earlier, a feature of CSMA/CD is simplicity. However, asnoted earlier, packet delays with CSMA/CD are unpredictable and highlyvariable, making conventional CSMA/CD unsuitable to support real-dimetraffic. In particular, backing off after several collisionssignificantly increases the latency suffered by a packet.

One variant of the Ethernet computer network, known as IsochronousEthernet, also transmits isochronous data but uses a frame form that isnot itself packetized. Thus, in Isochronous Ethernet, a special networkadapter is required that fragments packets into pieces and thentransmits each piece of a packet during a respective time slot ofprecise and fixed duration. Another specialized network adapter at thereceiving end then needs to reconstruct the packet from the pieces fordelivery to the device connected thereto. Thus, one drawback is thatsuch Isochronous Ethernet network adapters are not directly compatiblewith conventional Ethernet network hardware, so that special equipmentis required. There are no time periods wherein a regular Ethernet packetmay simply flow through a time slot on route. All Ethernet packets arefragmented and placed into multiple time slots. Another drawback is thatprecise synchronization and scheduling among the Isochronous Ethernetnetwork adapters are crucial for this type of network to functioneffectively. There is no CSMA/CD protocol withhn Isochronous Ethernet toavoid collisions should two nodes overlap in their timing. IsochronousEthernet uses only fixed-sized frames and time slots, so that networkbandwidth may be wasted should one or more slots not be utilized.

Additional mechanisms for providing isochronous channels within anEthernet network are described in U.S. Pat. Nos. 5,761,430 and5,761,431. While the mechanisms set forth in these patents may overcomesome of the drawbacks of Isochronous Ethernet by maintainingcompatibility with standard Ethernet, their utility for sending largevolumes of non-real-time computer traffic is limited by the requirementof timing and scheduling the transmission of all non-real time packets,as well as real-time packets. Furthermore, like Isochronous Ethernet,the mechanisms set forth in these two patents also require precisesynchronization corrections to be propagated throughout the network ineach frame. In sending such synchronization packets on a frequentper-frame basis, the large amount of time uncertainty and jitterinherent in Ethernet transmissions and computer interrupt processingactually introduce further synchronization errors and jitter at eachframe in these systems. And by requiring a reservation list to beincluded in each per frame beat packet, the resulting largersynchronization packet size for these conventional mechanisms furtherincreases the potential for timing jitter.

The mechanisms of these two patents further require the dynamicscheduling of packet transmission on a frame-by-frame basis according tothe presence or the absence of packets sensed per time interval on thenetwork. Therefore, these mechanisms become untenable for large numbersof independent sources of traffic, as au stations must correctly monitorall packets. Such a system does not scale well to networks with a largenumber of nodes. In addition, were any single station to encounter anerror in reading any of the broadcast packets, such a station could fallout of sync with the rest of the system.

In view of the foregoing, there is still a need in the art for networkapparatus and associated methodology that overcomes the limitations ofCSMA/CD and provides quality-of service guarantees in computer networksfor real-time traffic, while still maintaining full compatibility andutility for non-real-time traffic.

SUMMARY OF THE INVENTION

The present invention provides network apparatus and associated methodsfor minimizing or substantially eliminating unpredictable delays innetworks, particularly broadcast or Ethernet networks. One aspect of thepresent invention is its ability to create virtual isochronous channelswithin a CSMA/CD Ethernet network. The present invention provides anarbitration mechanism to control access to the network fortime-sensitive signals and to minimize or substantially eliminatecollisions. In an Ethernet network, this arbitration mechanism of theinvention augments the underlying CSMA/CD arbitration mechanism.

At regular intervals (or “frames”), dedicated time slots (or “phases”)are defined during which real-time traffic may be transmitted. Aplurality of network devices of this invention are synchronized togetherto define such frames to coincide on well-defined, periodic boundaries.This invention also provides an associated synchronization mechanismthat minimizes jitter and timing uncertainty of frame and phaseboundaries. The arbitration mechanism allows the real-time traffic toarrive at its destination with a very low and predictable delay. Theintroduction of predictability and a tight bounding on the delay allowsthe network to set guarantees for service quality.

According to one aspect of the present invention, a network forcommunicating packets of data includes a plurality of devices, forexample, real-time and non-real-time devices, and a network medium. Aplurality of device adapters connects the devices to the network medium.Each device adapter includes a device interface connected to one of thedevices and for receiving packets generated thereby and a networkinterface connected to the network medium. Each device adapter alsoincludes a processor connected to each of the interfaces for receivingthe packets from the device interface and for transmitting the packetsto the network interface.

One of the plurality of device adapters may serve as a master timingdevice that synchronizes a common time reference of the plurality ofdevices, Alternatively, a master timing device may be incorporatedwithin a specialized Ethernet repeater hub. The common time referencedefines a frame of time which, in turn, has a plurality of phases andrepeats cyclically. Each of The phases is assigned to a respectivedevice adapter. More than one phase can be assigned to a given deviceadapter. Each of the device adapters is allowed to transmit the packetsreceived at the device interface during the phase assigned thereto.Accordingly, as no device adapter is able to transmit packets out ofphase, collisions are eliminated for packets transmitted in the assignedphases. Furthermore, if a synchronization mismatch occurs, theunderlying CSMA/CD protocol intercedes to sense the transmission of apacket in a prior phase and to dynamically hold off transmission of apacket from a succeeding phase so as to prevent a collision. There areno collisions so long as the phase overlap does not exceed the timeduration of a minimum-sized packet. Another advantage is that thepackets do not need to be reformatted after transmission, so thatcompatibility with standard Ethernet is maintained.

The plurality of phases may also include a free-access phase, common toall connected device adapters, during which any of the device adaptersis able to transmit packets according to, for example, the standard IEEE502.3 CSMA/CD protocol. The device adapters may use information storedin a header of a packet received from an attached device to determinewhether to forward a received packet in an assigned phase, or as anon-real-time packet in the common free-access phase. If a packet issent in an assigned phase, service quality is guaranteed for the packet.Otherwise, if a packet sent in a free-access phase, the packet contendsfor network access along with all other device adapters.

The plurality of phases may also include one or more guard phases duringwhich none of the device adapters is able to transmit packets. A guardphase compensates for variations in signal delays between the deviceadapters. The optional use of a guard phase and CSMA/CD protocol, evenamong assigned phases, eliminates the need for precise synchronization.Should the transmission time of a first packet extend beyond itsassigned phase or a following guard phase, the device adapter associatedwith the next assigned phase senses this transmission and deferstransmission of a second packet until the first packet transmission iscompleted.

No collisions occur among packet transmissions during assigned phases solong as the device adapters synchronize their phases to within asynchronization tolerance time. This synchronization tolerance time iscalculated as the duration of a minimum-sized packet. In the case wherea first device adapter sends a first packet within its assigned phaseand a second device adapter attempts to transmit a second packet in asubsequent phase, this tolerance assures that the CSMA/CD mechanism willsense the first packet and delay transmission from the second deviceadapter sending the second packet until the first packet transmissionhas been completed. Thus, device adapters of this invention only need tobe in substantial synchronization and not precise or exactsynchronization. Furthermore, a guard phase at the start of a new framemay provide a settling period for any queued packets from the priorfree-access phase to ensure that a synchronization signal or a packetfrom the first assigned phase does not experience collisions.

Each of the phases has a pre-assigned length of time that may vary inproportion to the number of packets scheduled for transmission at thedevice interface of a respective the device adapter. Accordingly, if aparticular device connected to a device adapter is not generating alarge number of packets, then the phase assigned to that device adaptermay be shortened to eliminate idle time on the network. On the otherhand, if a particular device generates a large number of packets, thenthe phase assigned thereto may be lengthened to accommodate the largetraffic. Furthermore, a device adapter is able to use any unused time inan assigned phase that may otherwise be wasted to transmit non-real-timetraffic and thereby improve network efficiency of this invention.

The network of the invention may include a plurality of real-timedevices, such as telephones, and non-real-time devices, such ascomputers. The non-real-time devices may include a number nativenon-real-time devices connected to the network medium directly. Whenthere is a surplus of time to meet deadlines for real-time devices, thetransmission of real-time packets may be delayed in deference tonon-real-time packets generated by the native non-real-time devices.However, collisions may be forced for non-real-time packets when ascheduled real-time packet may otherwise miss a deadline.

Another aspect of the present invention is the underlyingsynchronization mechanism. This synchronization mechanism may utilizethe availability of inexpensive and stable crystal oscillators (XO). Thecrystal may be a variable crystal oscillator (VXO) with a narrow rangeof frequency adjustment, although this is not a requirement forachieving adequate synchronization according to the invention. The XO orVXO operates primarily as a free-running oscillator wherein theaccumulated phase mismatch is corrected via an occasional incomingtiming signal. When using a VXO, a separate VXO frequency correctionsignal is generated from the aggregate of many timing-signal phasemismatch measurements to tine-tune the VXO frequency. When using an XO,frequency correction can be achieved through periodic incremental phaseadjustments. One of the device adapters may be designated as the mastertiming device. In this case, the other device adapters, called slavedevices, synchronize their internal clock to the master timing sourcedevice. Alternatively, the master timing device may be incorporated intoa specialized Ethernet repeater hub. In is latter case, all of theattached device adapters function as slave devices and synchronize theirinternal clock to the master timing source device.

The drift and native frequency mismatch of the slave crystal oscillators(operating under a null correction voltage) with respect to the mastersets an upper bound on the frame length. The amount of phase drift whenoperating with no correction voltage must be small in relation to aminimum packet transmission time. In a preferred embodiment of theinvention configured for an Ethernet environment, this phase-drifttolerance typically is on the order of an Ethernet interpacket gap (IPG)over a period of many frame times, typically 10 or greater. Thus, havinga correction signal occur within this number of frames synchronizes thecommon time reference to within an IPG time. The VXO approach of thisinvention restricts frequency adjustment to a narrow range, usesregression techniques to account for variations in network delays in thedetermination of the magnitude of the correction, and separates thephase synchronization from the frequency fine tuning.

Another aspect of the present invention is that the synchronizationmechanism may use two types of synchronization signals: a fineresolution synchronization signal and a coarse-resolutionsynchronization signal. The fine resolution synchronization signal ofthe present invention need not carry any explicit information, andinstead conveys information implicitly through its arrival time. Fineresolution synchronization signals are sent at fixed times relative tothe time reference of the master timing source, for example, at thebeginning of a frame as defined by the master timing source. In thiscase, the arrival of the fine resolution synchronization signal at adevice adapter triggers a phase-synchronization event at said deviceadapter, adjusting the next frame boundary if necessary to coincide withthe arrival time of the fine resolution synchronization signal plus thenominal duration of the frame. However, the coarse resolutionsynchronization signal, which is in the form of a frame time-stamppacket, contains a full count of the current time at which the packet issent, relative to the master timing device. A coarse resolutionsynchronization signal can therefore arrive at anytime during the frameto which it refers. If used in conjunction with a fine resolutionsynchronization signal, the time stamp carried by a coarse resolutionsynchronization signal need only be precise enough to resolve thecurrent time to within a duration of a frame. The fine resolutionsynchronization signals, if used, may either be sent via the mastertiming source or delivered to the device adapters through some externalmechanism. The aspect of the present invention of a plurality offixed-length phases, each given phase being available for the entireduration of its associated isochronous stream, enables the use of TimeDivision Multiplexing (TDM) as a scheduling mechanism. By predeterminingthe length of each phase and the streams to which each phase belongs,the TDM scheduling of the present invention assigns isochronous streamsto specific phases. This simplifies implementation and robustness byintroducing predictability to a system. In TDM, a preset set of timescan be broadcast and used to time all packet transmissions.

Advantages of the present invention over conventional approaches forhandling real-time traffic include:

compatibility with conventional network devices operating under the IEEE502.3 standard Ethernet specification;

use of the CSMA/CD media access of IEEE 802.3 for self-adjustment ofphase mismatches to further prevent collisions among real-time packets;

ability to provide real-time service guarantees without monitoring ordynamic scheduling of real-time traffic; and

synchronization stability over many frames without the requirement forfrequent (per frame) resynchronization.

As a result, devices of the present invention can co-exist in systemsincorporating conventional Ethernet interfaces and will not adverselyaffect an existing network. For example, since the device adapters ofthis invention do not need to monitor real-time traffic, the deviceadapters can be used with standard switches and routers, as well asstandard repeater hubs. Furthermore, the specialized Ethernet repeaterhubs of this invention can be used with standard Ethernet devices.

Other aspects, features, and advantages of the present invention willbecome apparent to those persons having ordinary Hilt in the art towhich the present invention pertains from the following descriptiontaken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a is a schematic view of a conventional-Ethernet network;

FIG. 1 b is a schematic diagram illustrating a CSMA/CD arbitrationmechanism in a conventional Ethernet network;

FIG. 2 is a schematic view of an exemplary Ethernet network inaccordance with the present invention, particularly illustrating aConditioned Mode of the network, in which real-time devices andconventional Ethernet devices are attached to the Ethernet network;

FIG. 3 is a block diagram of an exemplary device adapter of the presentinvention wherein two Ethernet ports, one dedicated to non-real-timetraffic and another dedicated to real-time traffic, are mixed onto athird port that conditions an Ethernet link to allow a mixture ofreal-time and non-real-time traffic;

FIG. 4 is a graphical view illustrating the organization of time intorepeating frames and time intervals within each frame that defineallowable phases for each device to transmit time-sensitive traffic(Conditioned Mode);

FIG. 5 is a graphical view illustrating an arbitration mechanism inConditioned Mode o the invention, particularly illustrating thearbitration mechanism in which the duration of each phase is fixed;

FIG. 6 is a block diagram of an exemplary specialized Ethernet repeaterhub of the present invention, which repeater hub includes a means forgenerating and transmitting synchronization signals to the deviceadapters.

FIG. 7 is a schematic view of an exemplary Ethernet network inaccordance With the present invention, particularly illustrating anAnnex Mode of the network, in which real-time devices and conventionalEthernet devices are attached to the Ethernet network;

FIG. 8 is a graphical view illustrating the organization of time intorepeating frames and time intervals within each frame that defineallowable phases for each device to transmit time-sensitive traffice(Annex Mode);

FIGS. 9 a and 9 b are graphical views illustrating respective exemplaryarbitration mechanisms of the present invention in Annex Mode;

FIGS. 10 a, 10 b, 10 c, 10 d, 10 e, and 10 f are flowcharts illustratingrespective exemplary embodiments for packet transmission procedures fora Device Adapter of the present invention, covering both ConditionedMode and Annex Mode;

FIG. 11 is a block diagram of a specialized Ethernet repeater hubincorporating a master timing source and as ports for prior art Ethernetdevices;

FIG. 12 is a block diagram of a specialized Ethernet repeater hubincorporating a master timing source and associated configurableprocessor, as well as ports that can be configured to connect to eitherdevice adapters or prior art Ethernet devices;

FIG. 13 is a schematic view of an exemplary network in accordance withsome implementations; and

FIG. 14 is a schematic view of another exemplary network in accordancewith some implementations.

DESCRIPTION OF THE INVENTION

Referring to the drawings in more detail, an enhanced network 110 inaccordance with the present invention is illustrated in FIG. 2. As willbe discussed in more detail below, exemplary network 110 includes aplurality of devices 100 and 200 for generating real-time and/ornon-real-time packets of data for transmission across a network medium112 to a destination on the network 110. Exemplary network 110 alsoincludes a plurality of device adapters (Das) 1000 which ensure that atleast the real-time packets arrive at their destination withoutcolliding with other packets, thus guaranteeing a quality of serviceunavailable with conventional computer networks.

In addition to the hardware associated with the network 110, the presentinvention provides an arbitration mechanism to control access to thenetwork for time-sensitive signals and to minimize or substantiallyeliminate collisions. As discussed in more detail below, at regularintervals (or “frames”), dedicated time slots (or “phases”) are definedduring which real-time traffic may be transmitted. The arbitrationmechanism allows the real-time traffic to arrive at its destination witha very low and predictable delay. The introduction of predictability anda tight bounding on the delay allows the network to set guarantees forservice quality.

Continuing to reference FIG. 2, the plurality of device adapters 1000are connected to the network 110 at network interface points 2.Real-time devices (RTDs) 200, such as telephones and video equipment,are attached to the device adapters 1000. Non-real-time devices NRTDs)100, which are attached directly to network interface points inconventional networks, are preferably connected to the device adapters1000 in accordance with the present invention. The network 110 shown inFIG. 2 is configured in “Conditioned Mode,” as all traffic placed on thenetwork is conditioned by the device adapters 1000. The network includesanother mode, called “Annex Mode,” which will be discussed in moredetail below.

The network 110 may include a broadcast portion 1. The broadcast portion1 is an environment in which packets generated by one station aretransmitted to each of the stations on the network (i.e., packets arebroadcast throughout the network). Accordingly, collisions would occurin the broadcast portion 1 if the device adapters 1000 of the presentinvention were not present to control the transmission of packets. Thebroadcast portion I may be an Ethernet network or another type ofnetwork generally operating in a broadcast environment.

An exemplary embodiment of a device adapter 1000 of the presentinvention is illustrated in FIG. 3. Exemplary device adapter 1000includes a processor 1002 and a plurality of interfaces 1004, 1006, and1008. Interface 1004 is connectable to non-real-time devices 100;interface 1006 20 is connectable to real-time devices 200; and interface100 s is connectable to the network: 110. Each device adapter 1000 mayalso include a local clock 1010 such as a crystal oscillator and amemory 1012. The memory 1012 is connected to and controlled by theprocessor 1002. In addition to the embodiment shown in FIG. 3, thememory 1012 may be connected directly to the device interfaces 1004 and1006 or to the network interface 1008 for storing both real-time andnon-real-time packets prior to transmission.

As will be discussed in more detail below, the processor 1002 operatesin accordance with an arbitration mechanism that substantiallyeliminates collisions of real-time traffic. The device adapters 1000 maybe configured as stand-alone devices which may be connected to thenetwork medium 112, the realtime devices 100, and the non-real-timedevices 200. Alternatively, the device adapters 1000 may be configuredas adapter cards which may be inserted in expansion slots in, forexample, computers (illustrated as NRTDs 100 in FIG. 2) connected-to thenetwork 1.

The RTDs 200 may output data across a standard Ethernet interface.Conventional telephone and video equipment may be interfaced to thedevice adapters 1000 through an additional device which formats theoutput of the conventional equipment into Ethernet packets. Suchadditional formatting devices may be physically incorporated into thedevice adapters 1000.

To make efficient use of the broadcast medium 1 of the network 110,arbitration mechanisms of the present invention provide the capabilityof eliminating collisions and congestion in the network. This isaccomplished by establishing a common time reference among the deviceadapters 1000, and then using the common time reference to defineperiods of time when a particular device adapter has the exclusive rightto transmit packets on the network.

One exemplary arbitration mechanism of the invention for obtaining atime reference is to assign one of the device adapters 100 as a mastertiming device that transmits a synchronization signal at regularintervals or periodically to synchronize the local clock 1010 of eachadapter. Alternatively, as discussed in more detail later, the mastertiming device may be incorporated into a specialized Ethernet repeaterhub. The synchronization signal may be sent every predetermined numberof frames, such as every hundred frames at the so of a frame, or everypredetermined amount of time, such as 12.5 ms or 25 ms.

In addition, a slave device (i.e., a device adapter which is not themaster timing device) 20 may predict or measure the drift of its localclock 1010 with respect to the clock 1010 or time signal of the mastertiming device. The slave device may then use this drift measurement toadjust its local clock 1010 at regular intervals between synchronizationsignals from the master timing device. This technique allows the mastertiming device to transmit synchronization signals at less frequentintervals yet still adequately compensate for local oscillator drift.For example, if the local clocks 1010 are crystal oscillators, then theslave device may predict the drift with relative accuracy. If the driftis predicted to be about 60 us for every second, then for a frame havinga length of 25 ms, each slave device would adjust its local clock by I0.5 us per frame, or equivalently, by 60 μs after each 40 frames. If 60μs of clock mismatch are required, then this technique may significantlyextend the time interval between master synchronization signals to farlonger than one second. Or alternatively, this technique may provide fora significant-tolerance to loss or delay of a synchronization signal.

Alternative methods for obtaining a common time reference will bediscussed below. In the case where a standard Ethernet repeater hub 3 isused to interconnect device adapters, a master timing device may bedefined as the first of the device adapters 1000 to come on line. If amaster timing device goes off line, then a second of the device adapters1000 to come on line may become the new master timing device, and so on.

By definition, if at a given point in time the common time reference ist, then each of the device adapters 1000 knows the value oft to within abounded error e, and the absolute value of 10 the difference between theestimates of the common time reference at any two device adapters 1000is upper bounded by e. For purposes of explanation, it is helpful firstto assume that e=0 so that each device adapter knows the exact value ofthe common time reference.

Arbitration Mechanism

In contrast to conventional arbitration mechanisms, the presentinvention provides a mechanism in which repeating periodic frames aredefined. Each of the frames has an assigned section and an unassigned(or free-access) section. Access to the assigned section is regulatedand coordinated while access to the unassigned section is not. Theunassigned section may operate in accordance with conventional CSMA/CDEthernet protocol and may be used for the transmission of non-real-timepackets. The assigned section is synchronized, and transmission ofpackets during the assigned section is coordinated among all the otherdevices to eliminate collisions. The assigned section is primarilyreserved for real-time packets because such packets may be guaranteedwith a fixed delivery time or delivery within a deadline.

An exemplary arbitration mechanism of the present invention definesrepeating periodic time frames. Each time frame has an assigned (or“owned”) section and an unassigned (or “free-aecess”) section. Theassigned section is divided into a plurality of phases corresponding tothe plurality of device adapters 1000. Each of the phases is assigned to(that is, is owned by) one of the device adapters 1000. Each deviceadapter 1000 is allowed to transit packets of date, for example,real-time packets from RTDs 200, only during its assigned (or owned)phase, and is not allowed to transmit packets during the phase assignedto another device adapter. Accordingly, collisions between packets,particularly, real-time packets is eliminated. Each device adapter 1000,however, is allowed to transmit packets during the unassigned (orfree-access) phase. This exemplary arbitration mechanism will bediscussed in more detail below with particular reference to FIG. 4.

As mentioned above, the network of the present invention includes aplurality of device adapters 1000, which plurality is represented by N.The device adapters 1000 may then be respectively indicated by DA1, DA2,DA3, . . . DAN. Referring to FIG. 4, time is divided into equal lengthframes 20, 21, and 22 of duration F, for example, 25 ms. Only threeexemplary frames 20, 21, and 22 are show; however, the frames repeat ata periodic rate. For purposes of this discussion, an embodiment of thenetwork 110 includes four device adapters, i.e., N=4. Relative to thecommon time reference, the frame boundaries are at times t=nF, where nis an integer. Bach frame 20-22 is divided into N+1 non-overlappingintervals called phases, which phases are labeled p=1, 2, 3, . . . N+1.In the exemplary embodiment shown in FIG. 4, five phases 201, 202, 203,204, and 205 for the first frame 20 are shown.

Although each device adapter may own one or more phases, to simplify theexplanation of the operation of the present invention, we will take theexample where the first N phases are phases respectively owned by thedevice adapters 100, and which phases are generally indicated by numeral26. That is, if p satisfies 1≦p≦N 5 N, then phase p is owned by orassigned to DAp. A device adapter 1000 is not allowed to transmitpackets in any phase except for the phase owned thereby. That is, inthis example, device adapter DA1 only transmits in phase 1; deviceadapter DMA only transmits in phase 3; and so on. Accordingly,collisions are eliminated during owned phases. The network 110 is thensaid to be operating in Conditioned Mode. If real-time traffic istransmitted only during owned phases, then this arbitration mechanismeliminates collisions for real-time traffic, The device adapters 1000may store packets awaiting transmission during the assigned phases 26 inthe on-board memory 1012. Alternatively, such packets maybe stored inthe memory of the generating device 100 or 200 itself.

The assignment of phases 201-205 to the device adapters 1000 may becoordinated by a master scheduling device in response to requests fromthe other devices. The determination of which device adapter is to bethe master scheduling device may be analogous to the determination 30 ofthe master timing device discussed above; that is, the master schedulingdevice may be defined as DA1, with each device coming on linesubsequently respectively defined as DA2, DA3, and so on. If aspecialized Ethernet repeater hub is employed to interconnect the deviceadapters, a processor within the specialized Ethernet repeater hub mayserve as the master scheduling device. Alternatively, the masterscheduling device may not be a device adapter but may be another device,such as a computer, connected to one of the device adapters. The masterscheduling device may transmit a frame-start signal at the start ofevery frame 20, 21, 22, and so on. The number of phases in each framemay be defined or created by the master scheduling device in accordancewith the number device adapters 1000 that are on line. Accordingly, thenumber of phases may vary from frame to frame, and the length of eachphase may vary within a frame, as well as from frame to frame, inaccordance with the volume of packets to be transmitted by a particulardevice. The master scheduling device may broadcast this information tothe device adapters 1000 at the start of each frame. Alternatively, thephases may be of equal length with each device adapter 1000 choosing anunassigned phase by transmitting during the phase, thereby having thatparticular phase now assigned to the particular device adapter.

Each of the frames 20-22 may have a “guard” band or phase at the startof each frame during which no device adapter 1000 is allowed to transmitpackets. The guard phase accounts for variations in signal delays andvariability in quenching free-access transmissions from the previousframe. The guard phase will be discussed in more detail below.

With reference to FIG. 2, the network 110 of the present invention mayinclude bridges (switches) and routers. If included, then the bridgesand routers are used in place of or in conjunction with repeater hubs 3within the network. The time synchronization of the device adapters 1000can still function to eliminate congestion and contention at the bridge,thereby preserving deadlines and guaranteeing quality of service forreal-time signals. Furthermore, the aspect of the invention wherebyreal-time transmissions are pre-assigned phases at the time of the setupof a real-time or isochronous channel allows the invention to avoid themonitoring of the network for determining transmission times. Thispermits a network of this invention to utilize prior art bridges androuters, as well as bridges and routers incorporating device adapters ofthis invention.

If the latency of the bridges or routers is small with respect to theduration of a phase, then the traffic conditioning and real-timequality-of-service guarantees of the present invention will continue tofunction as described. If the latency of prior art bridges or routersis-substantial with respect to the duration of a phase, it may bedesirable to surround the prior art bridge or router with deviceadapters 1000. Alternatively, the device adapters 1000 of the inventionmay be physically and logically incorporated within a bridge or router.In this case, the device adapters subdivide the network into multipleconditioned domains for each side of a bridge or router wherein aseparate framing structure is used within each domain to continue toguarantee service quality. However, in this latter case, there may be atleast an additional frame of delay added to the overall latency forpackets crossing a conditioned domain.

With continued reference to FIG. 4, in addition to the owned or assignedphases 26, each frame 20, 21, 22 includes an unassigned, unowned, orfree-access phase which is indicated by numeral 27. The free-accessphase 27 is defined as phase N+1. The free-access phase 27 is defined asa phase in which any of the device adapters 1000 may transmit packets ofdata. Although the free-access phase 27 may be at any location withinthe frame, the free-access phase is shown in the drawings as the lastphase of a frame.

Arbitration within the free-access phase 27 may operate in accordancewith the CSMA/CD protocol. Therefore, collisions may occur during thefree-access phase 27. Each device adapter 1000 transmitting a packetduring the free-access phase may do so without crossing a frame boundary28. Thus, towards the end of the free-access phase, a device adapter1000 may have to refrain from transmitting a packet to ensure that itdoes not improperly transmit during the following phase.

Each of the phases 1, 2, 3, . . . N has a length of time indicated byχ₁, χ₂, . . . χ_(N) respectively. Time χ_(fa) is the length of thefree-access phase 27. As the length of each frame is preferablyconstant, as represented by F, then the summation of the lengths of thephases 26 and 27 equals the length of the frame, i.e., χ₁+χ₂+ . . .+χ_(N)+χ_(fa)=F.

An embodiment of the arbitration mechanism of the present invention isillustrated in FIG. 5. In this embodiment the lengths of the phases301-305 are constant across the frames. In describing the embodiment,four device adapters (i.e., N=4) are provided, for example. In eachframe 30, DA1 transmits two packets 31 and 32 during a first phase 301with each packet separated by an inter-packet gap (IPG) 19; DA2transmits a packet 33 during a second phase 302; DA4 transits a packet34 during a fourth phase 304, and two packets 35 and 36 are transmittedduring a fifth phase 205 separated by a collision 37. During phase p,DAp can transmit real-time traffic as well as non-real time traffic,where 1≦p≦N1. In this example, DA3 does not transmit any packets duringits assigned phase.

As mentioned above, each of the frames 30 may include a guard phase 300at the start of 5 the frame during which time no device adapter 1000 isallowed to transmit packets. If the device delays of the device adapters1000 are compensated. In addition, the guard phase 300 allows anypackets transmitted during the free-access phase 305 from the previousframe, which may not 10 have yet reached their destination, to bedelivered. Accordingly, the guard phase 300 is a period of time duringwhich no new packets are transmitted and the network 110 is essentiallyquiet. In the embodiment including the guard phase 300 at the beginningof each frame, the device adapters 1000 do not need to be preciselysynchronized but may operate somewhat out of synch and still guarantee ahigh quality of service in delivering real-time packets.

Another possible embodiment for an arbitration mechanism of the presentinvention is to eliminate the free-access phase 405, i.e., χ_(fa)=0, andto dynamically allocate the durations of the owned phases through atoken passing mechanism, as in some token ring protocols such as FDDI.

Synchronization

Referring to FIG. 2, according to an exemplary embodiment of theinvention, where a standard Ethernet repeater hub 3 is used tointerconnect the device adapters, one of the device adapters 1000 may bedesignated as a master timing device. Any of the device adapters 1000can be chosen as a master timing device. This master timing device maybe the same device adapter as the master scheduling device discussedabove or a different device adapter. Furthermore, the master schedulingdevice and/or the master timing device may not necessarily be deviceadapters, but some other device, such as a personal computer (PC),compatible with the device adapters of this invention and serving thepurposes of this invention. The selection of the master timing devicemay be determined through either an initialization protocol or a presetswitch setting. In a preferred embodiment, an initialization protocoluses a first-initialized-chosen scheme, wherein the first DA 1000 tocomplete initialization would be chosen as the master, preventing otherDAs from becoming a simultaneous master. Alternatively, a lowest mediaaccess control (MAC) address-chosen scheme may be used, wherein themaster is the device adapter with the lowest MAC address. Regardless ofhow the master is chosen, the protocol may also include a mechanism tochoose an alternate master. The alternate master becomes the master ifthe protocol senses that the primary (i.e., first-chosen) master hasgone off-line.

Alternatively, a specialized Ethernet repeater hub may be used tointerconnect the device adapters, which may assert itself as the mastertiming device. A specialized Ethernet repeater hub may also assertitself as the master scheduling device, Referring to FIG. 6, aspecialized Ethernet repeater hub 3 a in accordance with the presentinvention is illustrated with a block diagram. Such a specializedEthernet repeater hub 3 a may be used in place of a standard Ethernetrepeater hub 3 as in FIG. 2. As indicated in FIG. 6, a specializedEthernet repeater hub 3 a includes a standard Ethernet repeater hub 3, aprocessor 1020, an Ethernet interface 1022, and a clock source 1021. Theprocessor 1020 may obtain a time reference from the clock source 1021and use this to generate synchronization signals as discussed above.Such synchronization signals are sent as Ethernet packets to theEthernet interface 1022, which is connected to an Ethernet port 1024 aof the Ethernet repeater hub 3. Such synchronization signals are thendelivered to device adapters 1000 which are attached to other Ethernetports 1024 b-1024 g of the Ethernet repeater hub 3. The processor 1020may communicate directly with device adapters 1000, in order to serve asa master scheduling device as described above. Specialized Ethernetrepeater hubs 3 a may be interconnected with other Ethernet repeaterhubs using uplink ports 1023 to increase the number of device adaptersthat can attach to the network, which will become apparent to thoseskilled in the art.

In any case, upon selection, the master timing device sends two types ofsynchronization signals: a fine-resolution signal and acoarse-resolution signal. The fine-resolution signal is a frame-syncsignal that may be a packet or any other reliable and precise signalsource, either internal to or external from the network. It is notnecessary for the fine-resolution frame, -sync signal to carry anyexplicit information because a key characteristic thereof is its time ofarrival. It is preferable for the propagation time from the masterdevice to the slave devices to have minimal jitter and uncertainty inarrival time.

The synchronization mechanism may also compensate for propagation delayacross the network links. In one embodiment, the master timing devicesends a signal to a device adapter and instructs the device adapter toreturn the signal to the master timing device. The master timing devicemay then measure the round trip delay, dividing this by two, to derivean estimate of the propagation delay from the device adapter to themaster timing device. The master timing device may then send thisestimate to said device adapter so that said device adapter canappropriately compensate for propagation delay. By repeating thisprocess throughout the network, each device adapter may arrange forpackets sent thereby to arrive at the Ethernet repeater hub atdesignated times relative to phase definitions within a frame.Alternatively, each slave device adapter may directly measure thepropagation delay from a repeater hub thereto by sending a packet toitself by reflecting it off of the repeater hub. This technique allowseach device adapter independently to measure and calibrate asynchronization offset.

It may not always be possible to directly measure the round-trip time tothe source of the fine-resolution frame-sync signal, for example, whenthe source is external to the network. As discussed above, in apreferred embodiment, a specialized Ethernet repeater hub 3 a of thepresent invention may connect device adapters of the present inventionand provide the master timing source device. Time synchronizationmismatches may be compensated by a one-way transmission from each sourceDA to the master device adapter during a sync calibration cycle atsystem initialization. In his embodiment, each device adapter acts 1000as a slave device and transmits a sync verification signal to thespecialized Ethernet repeater hub 3 a. The specialized Ethernet repeaterhub then measures the time offset between the clock of each slave deviceand its local (i.e., master) clock and sends a correction offset valueback to the corresponding slave device. Thus, each slave deviceequalizes the phase delay from each slave device to the specializedEthernet repeater hub 3 a to facilitate precise coordination of TDMscheduled transmissions.

After phase alignment, any remaining phase mismatch between one DA andanother is small relative to a packet length. The underlying CSAM/CDmedia access protocol self-corrects for any such remaining phasemisalignments among the DAs. A phase misalignment may manifest itself asone DA attempting to transmit either too early or too late. If a DAtransmits too early, then the carrier sense of CSMA/CD suspends or holdsoff a transmission by a current phase until the transmission of theprevious phase completes, plus one IPG time. If a DA transmits too late,then wasted link capacity results for the idle gap because the previousphase may cause an overlap with a successive phase. If the misalignmentcauses a late transmission, a successive phase suspends or holds offtransmission by virtue of CSMA/CD. In neither case does a collisionoccur, as the TDM scheduling only permits a single source to transmit ina single phase.

In particular, a DA begins a packet transmission such that thetransmission would terminate at the end of the phase. However, phasemisalignment and possible delays in the start of transmissions due to acarrier sense hold-off may cause a transmission to carry over to thesuccessive phase. Therefore, according to an exemplary embodiment of theinvention, the start of the last packet transmission in a first phasepropagates across the network before the start of a second phase. Thispropagation takes place for the CMSA protocol, if necessary, to sensethe transmission from the first phase and to hold off the start of thesecond phase. By this means, the time multiplexing of this inventionself-aligns phase synchronization among all adjacent phases and therebyavoids collisions during the assigned phases.

The one-way transmission delay across an Ethernet network does notexceed 264 bit times and is typically less than 20 bit times for asimple star topology (for a background on such delay, see “The EvolvingEthernet,” Alexis Ferrero, Addison Wesley 1996, Chapter 10). Yet, aminimum sized Ethernet packet equals 512 bits plus a 64 bit preamble inlength. Before accounting for CSMA hold-off from a prior phase to add toany clock misalignment, there is a margin of between one half toapproximately the fall duration of a minimum-sized packet with respectto the master clock for device adapters of this invention to operate andstill avoid collisions during assigned phases. Thus, even afteraccounting for CSMA hold-off from a prior phase, or by simply extendingthe duration of a phase as compensation, device adapters of thisinvention can avoid collisions and guarantee transmission deadlines inthe face of significant clock misalignment.

Turning to the coarse-resolution signal, the master timing source devicebroadcasts the coarse-resolution signal as a frame time-stamp packet ona periodic but infrequent basis The frame time-stamp packet provides acoarse alignment of the current time. As the fine-resolution frame-syncsignal has already established a precise synchronization of frameboundaries, the coarse-resolution frame time-stamp packet can now arriveat the DAs at any time within the same frame as its transmission.

Over time, the phase of the clocks of the slave devices may start todrift from that of the master device. The arrival of the fine-resolutionsync signal realigns the phases. A measurement of the amount of phasedrift and the inter-arrival time of the fine-resolution sync signal alsocompensates for clock frequency mismatches and thereby creates afrequency compensation factor. Crystal oscillators typically have asmall frequency mismatch in accordance with manufacturing tolerances.Such mismatches, usually on the order of 100 parts per million (PPM),are adjustable with a variable crystal oscillator (VXO).

As mentioned above with reference to FIG. 3, according to an exemplaryembodiment of the invention, clock 1010 may be a VXO utilized as thetime source for each DA 1000. In such an embodiment, the master timingdevice does not adjust its frequency. However, each slave device usesthe frequency compensation factor of the tine-resolution sync signalfrom the master device to adjust the frequency of the VXO of the slavedevice to match the frequency of the VXO of the master timing device. Bycompensating for slave/master frequency mismatches, the fine-resolutionsync signal need only be broadcast at infrequent intervals. Thiscontrasts with conventional techniques that rely upon aphase-locked-loop ALL) having a voltage-controlled oscillator (VCO).Unlike a VXO, a VCO does not incorporate a crystal oscillator. Infree-running mode, a VCO may have a high degree of drift and jitter. ThePLL synchronization of the prior art relies upon a periodic beat packetarriving and mixing with a local VCO on each cycle of the oscillation tolock the frequency and the phase of the local clock to the arrival timeof the beat packet. However, each beat packet is subject touncertainties in interrupt processing and network transmission delays.These non-deterministic delays introduce random jitter to each local PLLVCO clock on a per-cycle basis. The resulting precise frequencysynchronization of the present invention creates a highly stablenetwork-wide time reference and greatly reduces clock jitter as comparedto prior-art PLL/beat timing source approaches.

Annex Mode

As mentioned above, in addition to Condition Mode, the network of thepresent invention operates in Annex Mode. With reference to FIG. 7, thenetwork operates in Annex Mode when the device adapters 1000 of theinvention coexist with prior art network interfaces called non-real-timedevices (NRTDs) that are attached directly to the network medium 112 vianetwork interface points 2, which devices are known as native NRTDs 101.The standard Ethernet repeater hubs 3 indicated in FIG. 7 may bereplaced with specialized Ethernet repeater hubs 3 a, in order toprovide a master timing device and possibly a master scheduling device.As discussed in more detail below, in Annex Mode, when there is asurplus of time to meet deadlines, the transmission of realtime packetsmay be delayed in deference to non-real-time packets. However,collisions may be forced for non-real-time packets when a scheduledreal-time packet may otherwise miss a deadline.

For example, a device adapter 1000 may determine whether there issufficient time to transmit and deliver a real-time packet by adeadline. If so, the device adapter may defer transmission of the packetto allow a native NRTD to transmit non-real-time packets. If not, thenthe device adapter may become aggressive in attempting to meet adeadline. The device adapter may transmit the packet to force acollision with the native NRTD. Or it may ignore the normal 502.3back-off algorithm and immediately retransmit after a collision withoutwaiting. Alternatively, the device adapter may retransmit before waitingthe full interpacket gap time to usurp media access; that is, the deviceadapter may reduce the interpacket gap and then immediately retransmitthe packet. Any combination of these techniques serve to increase thepriority of a device of this invention with respect to a native NRTD toguarantee timely delivery of a real-time packet transmitted by a deviceadapter in contention with one or more native NRTDs.

Exemplary network 110 may include a plurality of NRTDs 101 connecteddirectly to the Ethernet network 1 though network interface points 2.Real-time devices (RTDs) 200 may be attached to device adapters 1000,which in turn are connected to network interface points 2. The AnnexMode of operation of the network 110 is advantageous, as to support aconventional NRTD it is not necessary to connect the NRTD to a deviceadapter 1000, which means that a conventional Ethernet network can beupgraded incrementally as additional real-time devices are installed. Asillustrated in FIG. 7, NRTDs 100 are preferably attached to deviceadapters 1000 as the device adapters 1000 may condition the trafficgenerated by NRTDs 100 to reduce collisions. An NRTD that is directlyattached to a device adapter 1000 is considered a conditioned NRTD 100,and an NRTD that is directly attached to the conventional Ethernetnetwork is a native NRTD 101.

A central issue with Annex Mode of the network is that the native NRTDs101 may use a standard carrier sense multiple access collision detect(CSMA/CD) protocol and, hence, are not aware of any timing andpacket-pacing mechanism used by the device adapter. The device adapters1000 may support latency and throughput guarantees for real-time trafficby modifying the back-off protocol to ensure that packets from real-timetraffic are delivered in a timely manner, which will be discussed inmore detail below. However, as noted above, if a packet from a nativeNRTD 101 experiences several collisions, the latency suffered by thepacket significantly increases as the average delay grows exponentiallywith the number of collisions.

An arbitration mechanism of the present invention may support a moderatetraffic load from RTDs 200 without causing a significant increase in theaverage delay seen by native NRTDs 101, provided that the traffic loadoffered by the native NRTDs 101 is sufficiently low. R is preferable fornative NRTDs 101 to back off after collisions only when necessary tomeet deadlines of time-sensitive signals, or when congestion caused byother native RTDs 101 is present. As a native NRTD 101 does not knowwhen realtime traffic is being transmitted, this is not possible.Instead, the operation of the device adapters 1000 in Annex Modeprevents unnecessary collisions between device adapters 1000 and nativeNRTDs 101. The device adapters 1000 accomplish this goal by deferring tonative NRTD 101 traffic when possible.

The arbitration mechanism of the device adapters under Annex Mode willnow be described with reference to FIG. 8. As mentioned above, a commontime reference is obtained by the device adapters. Time is divided intoequal length frames of duration F, and frame boundaries occur at timest=nF relative to the common time reference, where n is an integer.Continuing the exemplary number of device adapters for this description,it is assumed that there are four device adapters 1000 (i.e., N=4). Eachframe is divided into N+1 non-overlapping intervals or phases, which arelabeled p=1, 2, 3, . . . N+1. Three frames 50, 51, and 52 are shown, andfive phases 501, 502, 503, 504, and 505 for frame 50 are shown. Thefirst N phases are owned by respective device adapters 1000, asindicated by numeral 56. That is, if p satisfies 1≦p≦N, then phase p isowned by DAp. A device adapter is not allowed to transmit in any ownedphase except for the phase that its own. However, as native NRTDs 101are oblivious to the framing structure, it is possible that native NRTDs101 will attempt to transmit a packet at any time during a frame.

Analogous to the discussion above, phase N+1 is unowned, as indicated bynumeral 57, is considered as a free-access phase, allowing any deviceadapter 1000 to transmit during this last phase of a frame. The CSMA/CDprotocol may be used during the free-access phase 57, and, therefore,collisions may occur during the free-access phase 57. Each deviceadapter 1000 transmitting a packet during the free-access phase 57 doesso without crossing the frame boundary 58. Thus, towards the end of thefree-access phase 57, a device adapter 1000 may have to refrain fromtransmitting a packet. Note that as native NRTDs 101 can transmit apacket at any time, a packet transmission from a native NRTD 101 maycross a frame boundary 58.

The length of the phases 501-505 may vary in each frame 50-52. At thebeginning of a frame with P owned phases, there are P numbers Y₁, Y₂, .. . Y_(P) known to the device adapters, such that 0<Y₁<Y₂< . . .<Y_(p)≦F. The interpretation of these numbers is that if a frame beginsat time t, then phase p of that frame ends at time t+Y_(P). Letting x₁,x₂, . . . x_(P) denote the lengths of phases 1, 2, 3, . . . P, in thisframe, respectively, then x₁+x₂+ . . . x_(P)=Y_(P) for all p satisfying1≦_(P)≦P. As discussed above before, as the length of each frame is theconstant F, the length of the free-access phase is x_(fa)=F−(x₁+x₂+ . .. +x_(P). In FIG. 8, it is assumed that P=for simplicity.

Exemplary arbitration mechanism utilized by the device adapters 1000 inAnnex Mode are illustrated in FIG. 9 a and FIG. 9 b. As mentioned above,a device adapter 1000 may only transmit packets during the phase it ownsor during a free-access phase. Thus, during phase p, the only devicesthat may transmit a packet are native NRTDs 101 and Dap. Also mentionedabove, native NRTDs 101 may use a CSAM/CD protocol. A native NRTD 101that is deferring transmission of a packet will typically wait only IPG19 seconds after sensing the network is idle before transmitting apacket, because if it were to wait longer, it would be at a disadvantagerelative to other devices implementing the CSMA/CD protocol. Ascollisions are most likely to occur after the network becomes idle, adevice adapter 1000 can avoid a collision with a native NRTD 101 bywaiting for a time longer than the IPG 19, namely, a defer timeT_(defer) 190 after sensing the network becomes idle before starting totransmit a packet. This gives native NRTDs the first opportunity to usethe network when the state of the network becomes idle, as shownillustrated FIG. 9 a as the possible timing of events during an ownedphase.

In this example, the transmission interval of a packet 61 transmitted bya native NRTDs 101 crosses the boundary 610 that defines the beginningof the phase. The DA 1000 which owns the phase has a packet 63 ready totransmit at the beginning of the phase 610, but defers (as indicated bynumeral 630) to two packet transmissions 61 and 62 from native NRTDs 101by waiting until it senses that the network is idle for a duration of atleast T_(defer) seconds. More specifically, a native NRTD 101 mayattempt to transmit a packet 62 during the transmission of packet 61,but as native NRTDs follow the CSMA/CD protocol and the network issensed busy, the native NRTD defers (as indicated by numeral 620) thetransmission until the channel is sensed idle for at least one IPG 19.

As the value of an inter-packet gap (IPG)19 is less than T_(defer), anative NRTD is able to begin the transmission of its packet 62 beforethe owner of the phase. In this example, the owner of the phase is firstable to transmit packet 63 after T_(defer) seconds (indicated by numeral66) following the end of the transmission of packet 62. In this example,after the owner of the phase transmits packet 63, the phase owner hasanother packet 65 ready to transmit. Similar to above, another nativeNRTD 101 transmits packet 64 after deferring (indicated by numeral 640)to packet 63 by waiting for at least IPG 19 seconds of idleness. Packet65 is not transmitted until T_(defer), seconds (indicated by numeral 67)after the end of the transmission of packet 64.

When a real-time packet needs to be fitted in order to meet a deadline,a device adapter 1000 may operate in a “aggressive mode,” whereby thedevice adapter waits for an inter-packet gap after sensing the networkbecomes idle before transmitting a packet. In addition, if a deviceadapter is involved in a collision while in the aggressive mode, thedevice adapter will not back off after the collision. As native NRTDs101 are required to back off after collisions according to conventionalCSMA/CD protocol, a device adapter 100 of the present inventionoperating in the aggressive mode can effectively monopolize the network,transmitting real-time traffic as necessary to meet deadlines. A deviceadapter 1000 will preferably operate in the aggressive mode only if thedevice adapter-would otherwise be in danger of delivering real-timetraffic later than required. In view of the foregoing, a device adapter1000 attempts to minimize the chances of collision with native NRTDs 101during the phase it owns. But when a particular device adapter isotherwise in danger of transmitting packets later than their deadlines,the device adapter may enter the aggressive mode.

An alternative approach for a device adapter operating in aggressivemode is to intentionally cause collisions with native NRTDs 101 withoutwaiting for packet transmissions to end. FIG. 9 b illustrates such anexample of the aggressive mode, illustrating a possible sequence ofevents during an owned phase. The first portion of the phase operates ina similar manner to that depicted in FIG. 9 a in that the transmissionof a packet 61 from a native NRTD 101 overlaps with the boundary 610that defines the beginning of the owned phase. Referring to FIG. 9 b, atthe beginning of the phase, the device adapter 1000 which owns the phasehas two packets 76 and 78 to transmit during the phase. However, as thephase owner is initially not in aggressive mode, the owner waits untilat least T_(defer), seconds of idleness are sensed on the network beforebeginning the transmission of a packet. Thus, a packet 72 from a nativeNRTD 101 is able to transmit a packet 72 after deferring (indicated bynumeral 720) to packet 71, and a packet 74 from a Native 10 NRTD 101 istransmitted after deferring (indicated by numeral 740) in the midst of acollision 73 that occurs between native NRTDs 101 after the transmissionof packet 72, due to simultaneous deference (indicated by numeral 730).

After transmission of packet 74, the owner of the phase determines thatit cannot wait any longer 760 to transmit packets 76 and 78, and,therefore, enters the aggressive mode (indicated by numeral 7678). Inthis example, a native NRTD 101 defers (indicated by numeral 750) atransmission until IPG seconds after packet 74. As the owner has enteredaggressive mode at this time, the owner also has the right to transmitIPG seconds after packet 74 ends transmission; and in this example acollision 75 occurs. After this collision, the native NRTD 101 backs offwhile the owner does not back off. Therefore, the owner is able totransmit packet 76 immediately after the collision. After thetransmission of packet 76 by the owner, the owner attempts to transmitpacket 78, but a collision 77 occurs with a native NRTD 101 which wasdeferring to packet 76. The owner. does not back off after thiscollision 77 and is able to successfully transmit packet 78 immediatelyafter the collision.

Preferred Embodiment for Transmission Processing

A preferred embodiment for managing packet transmissions by a particulardevice adapter 1000 is described-hierarchically in the flowchartsillustrated in FIG. 10 a-10 f. It is assumed that there are a total of Ndevice adapters 1000 in the network, and each device adapter 1000 isassigned a unique integer address q in the range 1≦q≦N. It is alsoassumed that each device adapter has an address p. The overallprocessing flow for a device-adapter is illustrated in FIG. 10 a. Thoseskilled in the art will understand that the flowcharts of FIGS. 10 a-10f are for illustrated purposes and that there are multiples offunctionally equivalent hardware and software implementations thereof.

The processing disclosed in FIGS. 10 a-10 f handles both the Annex andConditioned modes of the invention. Description of the network operatingunder Annex mode will be provided initially. As discussed in more detailbelow, the network operating under Conditioned mode can be achieved bymodification of a single parameter.

A frame begins at time t=nF, relative to the common time reference inthe local network, where F is the frame length and n is an integer. Avariable curren_-time is defined to hold the estimate of the common timereference of the device adapters, Thus, current_time increases at rateof real time, and the value of current_time across different deviceadapters 1000 is synchronized to within a small error. For purposes ofthis description, timing errors are ignored in FIGS. 10 a-10 f, withmodifications to accommodate timing errors later being discussed below.As mentioned above, if a frame starts at time t, then phase q withinthat frame ends a time t+Y_(q).

Transmission Processing Overview

Referring to FIG. 10 a, at the beginning of a frame 5001 the processingmoves to block 5010, wherein a counter named current_phase isinitialized to 1, and a variable named frame_start is loaded with thevalue current_time. The value of frame_start thus holds the time atwhich the current frame began. The value of current_phase represents theindex of the phase within a frame and is incremented accordingly as thevarious phases within a frame progress. From block 5010, the processingmoves to decision block 5020.

Within decision block 5020, the value of current_phase is compared tothe device adapter address p. If the quantities are not equal, theprocessing moves to decision block 5030, where the value ofcurrent_phase is compared to N+1. In this case, if the current_phase isnot equal to N+1, then this indicates that the system is in an ownedphase owned by another device adapter. Accordingly, in this case, theprocessing proceeds to the entry point 5405 of processing block 5400.The basic function of block 5400 is to silently wait for the end of thecurrent phase. When the end of the current phase is reached,current_phase is incremented by 1 with the bloc 5400 and the exit point5495 is reached. The details of processing block 5400 will be describedin more detail below.

Referring back to decision block 5030, if current_phase N+1, then thisindicates that the system is in the free-access phase, and theprocessing accordingly moves to the entry point 5105 of processing block5100. The function of processing block 5100, which will be described indetail later, is to manage packet transmissions according to standardEthernet CSMA/CD protocol while inhibiting transmissions at the end ofthe free-access phase, at which time the processing leaves block 5100through transition 5199 to the entry point 5405 of the processing block5400. In this case, within block 5400, the device adapter waits for thefree-access phase to end, increments current_phase, and exits at point5495.

Referring back to decision block 5020, if current_phase=p, then thisindicates that the beginning of phase p, which is owned by the deviceadapter, has started. Accordingly, the processing moves to the entrypoint 5205 of processing block 5200. The function of the processingblock 5100, which is also described in more detail below, is to transmitpackets during the phase owned by the device adapter. The transmissionswithin block 5100 will be done in a non-aggressive mode, deferring tonative device adapters by using a longer inter-packet gap. If the deviceadapter is able to transmit the required number of real-time packetsbefore the time D that phase p ends, namely, at time t+Y_(P), then thedevice adapter may transmit any queued non-real-time packets until thephase end time. At phase end, it then leaves the processing block 5200through the normal exit point 5295.

If the device adapter has no packets to transmit during phase p, theprocessing moves through transition 5298 to the entry point 5405 ofprocessing block 5400. In this case, within block 5400 the deviceadapter remains silent which signals the end of phase p, incrementscurrent_phase, and exits at point 5495.

If, during the course of phase p, the device adapter would otherwise bein danger of not being able to transmit real-time packets before theirdeadlines, the processing moves through transition 5299 to the entrypoint 5305 of processing block 5300. The function of processing block5300 is to transmit packets during the phase owned by the device adapteroperating in the aggressive mode. When the required number of real-timepackets have been transmitted during phase p, the device adapterterminates aggressive mode and leaves the processing block 5300 throughthe normal exit point 5395.

Under nominal operating conditions, a particular device adapter will beable to send all the required packets during phase p. However, as asafety measure, the processing may move through transition 5399 to theentry point 5405 of processing block 5400. In This case, the processingwithin block terminates phase p at the required time and current_phaseis incremented by 1 before moving to the exit point 5495 of processingblock 5400.

After the termination of a phase, at exit points 5295 or 5395, theprocessing moves to the decision block 5020 again, so that the nextphase within the frame can be processed. After termination of a phase atpoint 5395, the processing moves to decision block 5090. Withindecision, block 5090, the value of current_phase is compared to N+2. Ifcurrent_phase=N+2, this indicates the end of a free-access phase, whichis the last phase of a frame. The reason that current_phase=N+2 m thiscase is that current_phase is incremented from its value of N+1 withinprocessing block 5400. Accordingly, if current_phase=N+2 within block5090, then the processing moves through point 5099, indicating the endof a frame, to point 5010 where current_phase is reinitialized to 1 andthe frame processing repeats for the next frame. If current_phase is notequal to N+1 within decision block 5090, then The processing moves todecision block 5020 so that the next phase within the current frame canbe processed.

Block 5400: Waiting for Phase to End

Turning to the description of processing block 5400, reference is madeto FIG. 10 e. As mentioned above, the function of block 5400 is todetermine when the end of the current phase occurs, and incrementcurrent_phase by 1 when the phase transition occurs. From the entrypoint of the block 5405, the processing moves to decision block 5410wherein the value of current_time is compared to the sum of frame_startand Y_(current) _(—) _(phase). As mentioned above, by definition if aframe starts at time t, then phase q within that frame ends at time t+4.The purpose of the decision block 5410 is therefore when the currentphase ends. Accordingly, if current_-time is greater than or equal tothe sum of frame_start and Y_(current) _(—) _(phase), then the currentphase terminates and the processing moves from 5410 to 5445, where thevariable current_phase is incremented by 1. If current_time is less thanthe sum of frame_start and Y_(current) _(—) _(phase), then the phasecontinues until time frame_start+Y_(current) _(—) _(phase). Accordingly,the processing repeatedly reenter decision block 5410 until such timethe processing moves to block 5445.

Block 5200: Transmission of Packets During Owned Phase Non-Aggressively

Reference is made to FIG. 10 c for discussion of the processing withinblock 5200. As mentioned above, the function of block 5200 is to managethe transmission of packets during the phase that a particular deviceadapter owns. From the entry point 5205, the processing moves todecision block 5210, wherein it is determined whether the particulardevice adapter has any packets to be sent during phase p which it owns.If not, the processing moves through transition 5298 to the entry point5405 of processing block 5400, wherein the phase is terminated at theappropriate time as described above. If the particular device adapterhas packets to transmit during phase p, the processing moves to block5215. Within block 5215, the timer idle_timer is set to the parameterIPG_LOCAL. Once set to a positive value, idle_timer decrements at therate of real time until it reaches zero, at which time idle_timerretains the value zero until reset again. The parameter IPG_LOCAL isequal to a value longer than the standard interpacket gap IPG. Withinblock 5200, the device adapter attempts to avoid collisions with nativeNRTDs by waiting until the bus is sensed idle for IPG_LOCAL seconds.

Also within block 5215, a variable time_needed_rt is updated. The valueof time_needed_rt may be set equal to the maximum time it would take thedevice adapter to successfully transmit all the remaining real-timepackets that are required to be sent during the current phase, assumingthat the device adapter does so in the aggressive mode. Thus, thisincludes transmission times of such packets, as well as the maximum timewasted during collisions with native NRTDs, which collisions arerequired to cause the native NRTDs to back off and remain silent. Thespecification of the maximum time required by the device adapter totransmit the remaining real-time packets in the aggressive mode may beselected in accordance with a particular network implementation. Thevariable time_needed_rt is updated so that it can later be determined ifthe device adapter should enter the aggressive mode.

Upon leaving block 5215, the processing moves to decision block 5220,wherein the device adapter determines whether to send any more packetswith the current phase p. This includes real-time packets as well asnon-real-time packets. If not, the processing moves to the entry point5405 of processing block 5400, wherein the phase is terminated at theappropriate time as described above. If within decision block 5220 it isdetermined that the device adapter wishes to transmit more packetsduring the current phase p, the processing moves to decision block 5230.

The processing may traverse the cycle of blocks 5230, 5240, 5245, and5230, or may traverse the cycle of blocks 5230, 5240, 5250, and 5220until the time that the device adapter observers at least IPG_LOCALseconds of silence on the bus, or the time it must enter the aggressivemode. Specifically, within block 5230 the sum of current_time andtime_needed_rt is compared to the time when phase p must end by, namely,frame_start+Y_(P). If current_time÷time_needed_rt is greater thanframe_start+Y_(P), ten the device adapter enters the aggressive mode,and the processing moves through transition 5299 to the entry point 5305of process block 5300. If, on the other hand,current_time+time_needed_rt is less than or equal to frame_start+Y_(P),ten the device adapter can still attempt to transmit packets in thenon-aggressive mode. Accordingly in this case, the processing moves todecision block 5240, wherein the device adapter checks the state of thebus. If the bus is not idle, the processing moves to 5245 whereidle_timer is reset to IPGL_LOCAL, and the processing loops back todecision block 5230. If the bus is idle within block 5240, then theprocessing moves to block 5250, where the value of idle_timer iscompared with zero. If idle_timer is not equal to zero, then thisindicates that the device adapter has not yet observed IPG_LOCALcontiguous. seconds of silence, and the processing loops back todecision block 5230. If idle_timer is equal to zero within block 5250,then this indicates that the device adapter has observed IPG_LOCALcontiguous seconds of silence, and that the device adapter is nowenabled to send packets. Accordingly, in this case the processing movesto block 5275, wherein a packet is transmitted.

If the device adapter has real-time packets to transmit, the deviceadapter will attempt to transmit such packets before attempting totransmit any of the non-real-time packets it may have to transmit.

After transmitting a packet in block 5275, the processing loops back toblock 5215 in order to possibly transmit more packets, Aver the start ofthe packet transmission in block 5275, there are two possibilities.First, it is possible that the transmission collides with that of anative NRTD. In this case, the transmission is aborted after thecollision is detected, and the device adapter transmits a jam signal sothat all stations can reliably determine that a collision occurred. Asthe transmission is aborted, the value of time_needed_rt will not changein block 5215. If the transmission by the device adapter in block 5275is successful, then if it was a real-time packet, the variabletime_needed_rt is decremented in block 5215.

5 Block 5500: Management of Interpacket Gap Timer

FIG. 10 f illustrates a process which runs on a device adapter runs onDA concurrently with the main process described in FIGS. 10 a-10 e. Thepurpose of the process is to maintain a timer variable named IPG_timer.As indicated in the figure, the state of the bus is continuouslymonitored in decision block 5510. Whenever activity is sensed on thebus, the timer_IPG timer is set to a predetermined interpacket gap(IPG), which may be the value of the standard interpacket gap in theEthernet access protocol. While positive, the value of IPG_timer isdecremented at the rate of real-time until a value of zero is reached.Once zero is reached, IPG_timer remains constant until reset to apositive value. Thus, if IPG_timer equals zero at any point in time,then this indicates that the device adapter has observed silence for atleast the past IPG seconds relative to the current time.

Block 5300: Transmission of Real-Time Packets in Aggressive Mode

The process block 5300 is described with reference to FIG. 10 d. Asmentioned above, the function of block 5300 is to control the timing ofthe transmission of real-time packets by the device adapter in theaggressive mode during phase p. Upon entering the block through entrypoint 5305, the processing begins at decision block 5310, where thevalue of IPG_timer is compared with zero. If IPG_timer is not equal tozero, then the processing loops back to decision block 5310. Theprocessing does not break from decision block until IPG_timer is equalto zero. When IPG_timer is equal to zero, this indicates that IPGseconds of silence have elapsed, and accordingly a packet transmissioncan start. Accordingly, in his case the processing moves to block 5320.

Within decision block 5320, a variable tx_time_next is referenced. Thisvariable holds the transmission time of the next real-time packet to betransmitted during the current phase. The sum of current_time andtx_time_next is compared to frame_start+Y_(P). Ifcurrent_time÷tx_time_next is greater than frame_start+Y_(P), thentransmission of the next real-time packet that requires transmission inthe current phase would cause the duration of phase to extend beyondtime t+Y_(P), which violates the constraint on the ending time of phasep. Accordingly, in this case, the processing moves through transition5399 to the entry point 5405 of block 5400, so that the current phasewill terminate as required. The transition 5399 is included as a safetyvalve to ensure that phase p terminates by the required time and willnot be traversed under nominal conditions. If current_time+tx_time_nextis less than or equal to frame_start+Y_(P), then there is sufficienttime to transmit the next real-time packet within the current phase p,and the processing moves to Block 5345, wherein a real time packet istransmitted.

After the packet has begun transmission in 5345, the processing moves todecision block 5340. There are two possibilities for the fate of thepacket transmission. If a collision occurs. the transmission is abortedas soon as the collision is detected, and a JAM signal is sent, as instandard Ethernet access protocol. In this case, the processing movesfrom 5340 back to decision block 5310, so that the packet can beretransmitted. The device adapter does not back off after a collisionbut instead may try to transmit after waiting only for the bus to remainsilent for the standard interpacket gap IPG. If the transmission inblock 5345 completes successfully, then the processing moves from block5340 to decision block 5350.

Within decision block 5350, the device adapter determines whether thereare more real-time packets remaining to be transmitted during thecurrent phase p. If so, the processing loops back to decision block5310, so that the remaining real-time packets may be transmitted. If notthe processing proceeds to the entry point 5405 of block 5400, so thatthe current phase will terminate as required.

Block 5 100: Transmission of Packets in Free-Access Phase

Referencing FIG. 10 b, an exemplary implementation of process block 5100is illustrated. As mentioned above, the function of block 5100 is totransmit packets during the free-access 25 phase according to standardCSMA/CD protocol of Ethernet, while inhibiting transmissions at the endof the phase. The processing enters decision block 5110 after passingthrough the entry point 5105.

Within decision block 5110, a variable tx_time_next is referenced. Thisvariable holds the transmission time of the next packet to betransmitted during the current phase, and is equal to zero if there isno packet currently queued. The sum of current_time and tx_time_next iscompared to frame_start+Y_(N+1). As described above, the free-accessphase within the current frame ends at time frame_start+Y_(N+1).Accordingly, if current_time+tx_time_next is greater than or equal toframe_start+Y_(N+1), then the next packet cannot be successfullytransmitted within the current free access phase, and the processingmoves through transition 5199 to the entry point 5405 of block 5400,where the free-access phase will be terminated as appropriate. Ifcurrent_time+tx_time_next_is less frame_start+Y_(N+1), then theprocessing moves to decision block 5120.

Once the processing moves to decision block 5120, it is allowable forthe device adapter to attempt transmission of a packet. However, it mustwait for at least IPG seconds of silence before doing so, and back offfrom any previous collisions that may have already been suffered by thepacket. Accordingly, within decision block 5120, the device adaptertests to determine whether IPG_timer is equal to zero and backoff_timeris equal to zero. If so, the device adapter has observed IPG seconds ofsilence and is through backing off from any previous collisions that mayhave occurred, and thus proceeds to decision block 5130. If not, theprocessing loops back to decision block 5110.

Within decision block 5130, the device adapter determines whether thereis a pack: et waiting to be transmitted. If not, the processing loopsback to decision block 5110. If so, the processing moves to 5140 and thepacket is transmitted.

After the packet has begun transmission in block 5140, the processingmoves to decision block 5150. There are two possibilities for the fateof the packet transmission, If a collision occurs, the transmission isaborted as soon as the collision is detected, and a JAM signal is sent,as in the standard Ethernet access protocol. In his case the processingmoves from 5150 to block 5170. Within block 5170, the timerbackoff_timer is set to a random retransmission delay as in the standardtruncated binary exponential back-off algorithm within the Ethernetprotocol. In particular, if a packet has experienced k collisions, thenbackoff_timer is set to iT, where T is the slot time and i is a randominteger in the range 0≦i<2^(m) and m=min{k,10}. After a packet hasexperienced 16 collisions, the packet is discarded. Note that as long asthe timer backoff_timer remains positive, backoff_timer decrements atthe rate of real time until it reaches zero. When zero is reached,backoff_timer retains the value of zero until reset to a positive value.Thus, when backoff_timer=0, the device adapter is through backing offfrom any previous collisions that may have occurred. If the transmissionin block 5140 was successful, then the processing moves from block 5140to block 5160, where the backoff_timer is set to zero. From either block5160 or block 5170, the processing loops back to decision block 5110 sothat the next transmission or retransmission can proceed if possiblewithin the free-access phase.

During the free-access phase, it may be preferable for the deviceadapter 1000 to use a longer interpacket gap, IPG_LOCAL, in order toavoid collisions with other device adapters 1000 and native NRTDs,thereby surrendering priority to native NRTDs. The necessarymodifications to process block 5100 in order to implement this will beapparent to someone skilled in the art.

Transmission Processing for Conditioned Mode

If the network is configured in Conditioned mode rather than Annex mode,then no collisions are possible during owned phases, so that it isunnecessary for a particular device adapters to defer by using a longerinterpacket gap within the phase p that it owns. In this case, theprocessing can be optimized by setting the parameter IPGL_LOCAL, definedwithin processing block 5200, to the standard interpacket gap IPG. Inthe conditioned mode, the process block 5300 will not be entered undernominal conditions. Preferably, a device adapter 1000 can automaticallydetect whether or not the network is configured in Conditioned mode orAnnex mode by detecting collisions during owned phases, for example, andset the value of IPG_LOCAL accordingly.

Universal Ethernet Repeater Hub with Prior Art Ethernet Ports

In addition to the Annex mode described above, the present inventionprovides alternative methods and apparatus for configuring bothreal-time devices (RTDs) 200 and non-real-time device (NRTDs)100 thatare connected to a device adapter (DA)1000 (see FIG. 7) withconventional non-real-time devices (NRTDs)101 into a network. In thisregard, an exemplary embodiment of a universal Ethernet repeater hub 3 bwith prior art Ethernet ports in accordance with the present inventionis illustrated in FIG. 11. Exemplary universal repeater hub 3 b, whichmay function as either a master timing device or a master schedulingdevice, eliminates collisions between native NRTDs 101 and deviceadapters. This is accomplished by determining whether a packetoriginates from a prior art device or from a device connected to adevice adapter 1000, as discussed in detail below.

Universal repeater hub 3 b includes a plurality of conventional Ethernetrepeater hubs 3, preferably two repeater hubs as shown. One of theEthernet repeater hubs 3 connects to native NRTDs 101, via Ethernet aplurality of ports 1036 b-1036 g, and the other Ethernet repeater hub 3connects to device adapters 1000 via a plurality of ports 1034 b-1034 g.As there are two separate Ethernet repeater hubs 3, packet transmissionsfrom both the device adapters 1000 and the connected native. NRTDs 101may be buffered, which is discussed in detail below. Exemplary universalrepeater hub 3 b includes a processor 1030 connected to the conventionalEthernet repeater hubs 3 via respective Ethernet interfaces 1032 a and1032 b. Accordingly, processor 1030 can independently communicate withdevices attached to either of the Ethernet repeater hubs 3.

Exemplary processor 1030 operates analogously as a device adapter 1000on behalf of the attached native NRTDs 101. In particular, packetsreceived from a native NRTD 101 may be temporarily stored in a memorydevice 1035 connected to the processor 1030 before being forwardedthrough port 1034 a of the Ethernet repeater hub connected with deviceadapters 1000. Such forwarding, through Ethernet interface 1032 a, ispreferably carried out in accordance with the condition mode of thearbitration mechanism described above. Conversely, packets received fromdevice adapters 1000 are forwarded through port 1036 a of the-Ethernetrepeater hub connected to the native NRTDs 101. Packet transmissions onEthernet interface 1032 b are preferably carried out in accordance withstandard CSMA/CD protocol.

Regarding buffering, a real-time packet received at one of the ports1034 of a first of the repeater hubs 3 (i.e., the repeater hub dedicatedto the device adapters) and addressed to a device connected to anotherone of the ports 1034 of the first repeater hub 3 is not buffered but israther repeated out of all the ports 1034 of the first repeater hub 3 totransmit the packet to the addressed device. However, if a real-timepacket received at one of the ports 1034 of the first repeater hub 3 isaddressed to a device connected to one of the ports 1036 of a second ofthe repeater hubs 3 (i.e., the repeater hub dedicated to conventionalNRTDs), then such a packet is buffered by the processor 1030 until thesecond Ethernet repeater hub is idle as per the CSMA/CD protocol.

In addition, a non-real-time packet received at one of the ports 1036 ofthe second repeater hub 3 and addressed to a device connected to one ofthe ports 1034 of the first repeater hub may be buffered by theprocessor 1030 until the next free-access phase, during which time sucha packet is repeated to each of the ports 1034 to transmit the packet tothe addressed device. During free-access phases, the repeater hubs 3essentially act as a single hub, with each incoming packet transmitteddirectly to the addressed device without the need to buffer the packets,for example, by broadcasting the incoming packets to each of the ports.

Exemplary universal Ethernet repeater hub 3 b may also include a clocksource 1031 so that the universal repeater hub 3 b can act as a mastertiming source as described above. Moreover, as described above, theprocessor 1030 can also serve as the master scheduling device. Inaddition uplink ports 1033 a and 1033 b of the Ethernet repeater hubs 3can be used to connect with additional repeater hubs (not shown) toprovide more ports for connecting with additional device adapters andnative NRTDs 101.

Universal Ethernet Repeater Hub with Configurable Forts

Another exemplary embodiment of the universal Ethernet repeater hubs ofthe present invention is illustrated in FIG. 12 and indicated byreference numeral 3 c. Exemplary universal Ethernet repeater hub 3 cincludes a plurality (e.g., a pair) of conventional Ethernet repeaterhubs 3 each with a plurality of ports. In contrast to the embodiment ofthe universal repeater hub 3 b shown in FIG. 11 in which two sets ofports (i.e., one for connected to device adapters and one for connectingto native NRTDs) are provided, exemplary universal repeater hub 3 cshown in FIG. 12 includes one set or type of port configured forconnecting to either a device adapter 1000 or a native NRTD 101. Thearchitecture of exemplary universal Ethernet repeater hub 36 shown inFIG. 12 is analogous to exemplary universal Ethernet repeater hub 3 bshown in FIG. 11 except for the inclusion of a plurality of ports 1045respectively connected to a plurality of switches 1050.

Each of the ports 1045 is connected to either a device adapter 1000 or aconventional NRTD 101. The switches 1050 select which of the Ethernetrepeater hubs 3 an attached device is connected to by determiningwhether a particular port 1045 is connected to a device adapter 1000 ora conventional NRTD 101. The switches 1050 may be controlled manuallybut are preferably controlled automatically. Manual control may beaccomplished with mechanical switches. The automatic control of theswitches 1050 may be accomplished electrically. Such electrical controlmay require additional hardware (not shown) to determine which type ofdevice a port is attached to. The requirements of such additionalhardware will become apparent to someone skilled in the art.

In accordance with the present invention, each of the switches 1050 inconjunction with the processor 1030 determines whether the port 1045corresponding thereto is connected to either a device adapter 1000 or aconventional NRTD 1101. If a port 1045 is connected to a device adapter1000, then all packets received at that port are directed to the firstof the repeater hubs 3 by the corresponding switch 1050. Conversely, ifa port 1045 is connected directly to a conventional NRTD 101, then allpackets received at that port are directed to the second of the repeaterhubs 3 by the corresponding switch 1050. The switches 1050 may determinewhether a port 1045 is connected to a device adapter 1000 by, forexample, having the processor 1030 send a timing signal or other specialpacket from the clock source 1031 to the device connected thereto asdescribed above. If an appropriate response signal is returned, then thedevice connected to that particular port is a device adapter; if nosignal is returned, then the device connected to that port is aconventional NRTD.

Dynamic Operation

In the arbitration mechanisms described above, each device adapter 1000in the network owned a phase in every Frame. If a device adapter 1000 isnot actively carrying any realtime traffic (e.g., a telephone is onhook), the device adapter may be desirable to de-allocate the phaseowned by this inactive device adapter. Using non-real-time packets, thedevice adapters 1000 may coordinate to agree on how many phases are ineach frame and on the ownership of the phases. Each device adapter 1000,active or not, may be periodically required to transmit a packetannouncing its existence. Each device adapter 1000 may then maintain atable of device adapter that have announced their existence, whichentries expire if a corresponding announcement is not heard before atimer expires. The addresses of the device adapters in this table thendefine a natural ordering between the device adapters 1000 in thenetwork, which can be used to define the order of ownership of ownedphases during a frame, and to define the master scheduling device.

In addition to Ethernet networks, the principles of the presentinvention may be applied in conjunction with networks operating inaccordance time division multiple access (TDMA) or synchronous opticalnetwork (SOME) protocols. For example, asynchronous transfer mode SONET(ATM/SONET) networks transmit large Frames with predetermined fixed timeslots at regular intervals. A SONET frame may be received on an OC3 lineby a device adapter 1000 and particular cells from the SONET frame maybe converted into or configured as a packet in an assigned phase of thepresent invention. For example, specific time slots of the SONET framethat have been assigned to a particular virtual channel may be assignedto respective device adapters from a remote Conditioned sub-network(i.e., a network connected to a device adapter 1000 of the invention).Accordingly, the device adapters 1000 of the present invention are notonly compatible with conventional network hardware but also providecompatibility across network protocols.

Systems and techniques described herein may be used with wirelessnetworks (e.g., WiFi and other CSMA wireless networks, TDMA networks,etc.), and with wired networks employing a variety of physical layers(e.g., Ethernet and SONET networks).

For example, the systems and techniques may be used with powerlinecommunication (PLC) networks, home phone line (NPN) networks (e.g.,networks conforming to Home Phoneline Networking Alliance (Home PNA)standards), and cable networks (e.g., quadrature amplitude modulation(QAM) networks).

FIG. 13 shows an exemplary network 1300. In an implementation, network1300 is a PLC network. PLC networks provide for data communication usingexisting powerlines rather than dedicated data communicationinfrastructure. Network J300 may provide data communication for a numberof devices 1320 via lines 1310 (in this implementation, powerlines).Network 1300 may be a local powerline communication network for alocation 1305 such as a house or building. Typically, network 1300 willbe in communication with the Internet 1330 through one of the devices1320 on the network.

Devices 1320 may each include a device adapter 1325 to receive data fromthe associated device 1320 and to transmit data onto the powerline 1310according to a frame of time including at least one assigned time phaseand a free access phase. The frame of time may be substantiallysynchronized among the device adapters 1325. Any of the device adapters1325 may transmit data on the network 1300 during an assigned time phasefor the particular device adapter 1325, or during the free access phaseor both. If any of the devices 1320 is not generating data fortransmission, its device adapter 1325 may be deemed inactive, and maynot have a time phase assigned.

In another implementation, network 1300 may be a home phoneline network(HPN), with lines 1310 being standard telephone lines. As above, deviceadapters 1325 may transmit data on the network 1300 during an assignedtime phase for the particular device adapter 1325, or during the freeaccess phase or both.

In another implementation, the current systems and techniques may beused in a cable network. FIG. 14 shows an implementation of a cablenetwork 1400 incorporating a hybrid fiber coax (HFC) medium. The HFCmedium supports two-way traffic by the modulation/demodulation of datasignals of frequency spectrum bands. For example, data signals may becommunicated on network 1400 by quadrature amplitude modulation (QAM) ofcarrier waves, quadrature phase-shift keying (QPSK), or other modulationschemes that encode groups of binary digits as modulated symbols. Acable plant 1430 delivers content (e.g., some or all of video, audio,data or other content) to users and may also receive content from users.

For example, cable plant 1430 may send content to and receive contentfrom one or more locations 1405 (e.g. homes, businesses, etc.). Thecontent may be transmitted to a multitude of neighborhoods through afiber ring distribution network 1435. The content may be transmitted aneighborhood distribution node 1440 to locations 1405 via coaxial cablelines 1410, which may be in a tree configuration as shown. Thiscombination of fiber distribution rings and last-mite coaxial cabledistribution to the home is generally referred to as a Hybrid Fiber-Coaxnetwork, or HFC. Note that there may be many users upstream of theparticular group of locations 1405 shown.

Each location 1405 may include one or more devices 1420 for generatingand/or receiving content. Each device 1420 may be in communication witha device adapter 1425 that is configured to receive content from thenetwork and provide it to device 1420, as well as to receive contentfrom device 1420 to provide it to the network.

In some existing systems, cable plant 1430 manages data according to aprotocol in accordance with Data Over Cable Service InterfaceSpecification (DOCSIS). In such a system, cable plant 1430 uses a TDMAsystem to govern transmission on the network by device adapters 1425, aswell as any upstream devices on the network.

Using the current systems and techniques, content may be transmittedfrom a device 1420 by a device adapter 1425 according to a frame of timeincluding at least one assigned time phase and a free access phase.Device adapters 1425 may transmit content during an assigned time phase,during the free access phase, or both. Non time-sensitive content (suchas spreadsheet data, etc.) may be sent during the free access phase.During the free access phase, collisions may be handled using (forexample), CSMA/CD to resend data.

During an assigned time phase, only one device adapter 1425 transmitscontent. Therefore, quality of service for time-sensitive content (suchas video or voice content) may be preserved by transmitting the contentduring a device adapter's assigned time phase. The frame of time may besubstantially synchronized among the device adapters 1425. If one ormore for the devices 1420 is not generating data for transmission, itsdevice adapter 1425 may be deemed inactive, and may not have a timephase assigned.

In contrast with a TDMA-only system, a hybrid system allows for moreefficient utilization of the network for the short, bursty, randomupstream traffic characteristics that typically characterize generalInternet web browsing usage. For example, a single spontaneous upstreampacket that would result from a click on a web link would no longersuffer the overhead and limitations of scheduling through the headendwhen using the fee-access mechanism described herein.

A number of benefits may be provided by managing data transmissionaccording to the current systems and techniques. Shared-medium networks,such as home phone line networks and cable distribution networks, incurlong duration startup periods due to ringing, slow rise-times, and otherforms of waveform degradation. A collision forces a repeat of thisstartup delay. Media access control (MAC) mechanisms, such as CSMA/CD,may be inefficient for networks in which significant ringing occurs,because the backoff times required to allow the ringing to subside to anacceptable level may be unduly long.

Assigned time phases such as described herein may thus allow forimproved throughput in such networks by avoiding such collisions.Ringing may be a particular problem in HPN and PLC networks, which aregenerally unterminated and run in a single bus for a particularresidence or business. Ringing and the associated increase in backofftimes may also be a problem for cable (e.g., QAM) networks, due to thetime sensitivity of content delivered over the networks.

As noted above although TDMA systems may avoid startup transients, theymay be inefficient for Internet data access some networks, such asDOCSIS cable networks using QAM or QPSK. Upstream transmission delaysfor TDMA DOCSIS networks may measure in the tenths of seconds to onesecond as a result of the wait required for a particular slot to comearound. Thus, a free access phase as described herein may significantlyshorten transmission delays and increase system responsiveness.

Those skilled in the art will understand that the embodiments of thepresent invention described above exemplify the present invention and donot limit the scope of the invention to these specifically illustratedand described embodiments. The scope of the invention is determined bythe terms of the appended claims and their legal-equivalents, ratherthan by the described examples. In addition, the exemplary embodimentsprovide a foundation from which numerous alternatives and modificationsmay be made, which alternatives and modifications are also within thescope of the present invention as defined in the appended claims.

1. A device adapter, comprising: a transmission unit configured totransmit data from a real time device via a network according to a timeframe, wherein the time frame is substantially synchronized in thedevice adapter and at least one other device adapter, the time framerepeating periodically and including a plurality of assigned time phasesand a free access phase, wherein the device adapter is configured totransmit data during at least one of an assigned time phase associatedwith the device adapter prior to transmission of data from the real-timedevice by the device adapter, and included in the plurality of assignedtime phases, or the free access phase, to refrain from transmitting dataduring time phases of the plurality of assigned time phases that are notassociated with the device adapter, and to be able to determine whetherto defer transmission of data during the assigned time phase associatedwith the device adapter and the free access phase to allow a non-realtime device to transmit data.
 2. The device adapter of claim 1, furtherconfigured to defer transmission of data for a predetermined periodwhich is longer than a defer transmission period implemented in astandard carrier sense multiple access collision detect (CSMA/CD)protocol.
 3. A device adapter, comprising: a transmission unitconfigured to transmit data from a real time device via a networkaccording to a time frame, wherein the time frame is substantiallysynchronized in the device adapter and at least one other deviceadapter, the time frame repeating periodically and including a pluralityof assigned time phases and a free access phase, wherein the deviceadapter is configured to transmit data during at least one of anassigned time phase or the free access phase, to refrain fromtransmitting data during time phases not assigned to the device adapter,and to determine whether to defer transmission of data during at leastone of the assigned time phase or the free access phase to allow anon-real time device to transmit data, and wherein the device adapter isfurther configured to defer transmission of data for a predeterminedperiod which is equal to a defer transmission period implemented in astandard CSMA/CD protocol and to continue the data transmission if acollision occurs.
 4. The device adapter of claim 1, further configuredto transmit data during data transmission by the non-real time device,thereby causing a collision, and to continue data transmission after thecollision.
 5. A method, comprising: obtaining, at a device adapter, asubstantially synchronized time frame which is maintained among aplurality of device adapters interconnected by a network, the time framerepeating periodically and including a plurality of assigned time phasesand a free access phase; controlling transmission of data during atleast one of an assigned time phase assigned to the device adapter priorto transmission of real time data by the device adapter, and included inthe plurality of assigned time phases, or the free access phase;refraining from transmitting data during one or more time phases notassigned to the device adapter; and determining whether to defertransmission of data during the assigned time phase assigned to thedevice adapter if the device adapter has data to transmit, during theassigned time phase assigned to the device adapter, to allow a non-realtime device to transmit data during the assigned time phase assigned tothe device adapter; and determining whether to defer transmission ofdata during the free access phase if the device adapter has data totransmit during the free access phase, to allow a non-real time deviceto transmit data during the free access phase.
 6. The method of claim 5,further comprising deferring transmission of data for a predeterminedperiod which is longer than a defer transmission period implemented in astandard CSMA/CD protocol.
 7. A method, comprising: obtaining, at adevice adapter, a substantially synchronized time frame which ismaintained among a plurality of device adapters interconnected by anetwork, the time frame repeating periodically and including a pluralityof assigned time phases and a free access phase; controllingtransmission of data during at least one of an assigned time phase orthe free access phase; refraining from transmitting data during timephases not assigned to the device adapter; determining whether to defertransmission of data during at least one of the assigned time phase orthe free access phase to allow a non-real time device to transmit data;and deferring transmission of data for a predetermined period which isequal to a defer transmission period implemented, in a standard CSMA/CDprotocol and continuing the data transmission if a collision occurs. 8.The method of claim 5, further comprising transmitting data during datatransmission by the non-real time device, thereby causing a collision,and continuing data transmission after the collision.
 9. Anon-transitory computer readable medium containing instructions that, inresponse to execution by a processor, result in the implementation ofoperations comprising: obtaining, at a device adapter, a substantiallysynchronized time frame which is maintained among a plurality of deviceadapters interconnected by a network, the time frame repeatingperiodically and including a plurality of assigned time phases and afree access phase; controlling transmission of data during at least oneof an assigned time phase assigned to the device adapter prior totransmission of real-time data by the device adapter, and included inthe plurality of assigned time phases, or the free access phase;refraining from transmitting data during one or more time phases notassigned to the device adapter; and determining whether to defertransmission of data during the assigned time phase assigned to thedevice adapter if the device adapter has data to transmit during theassigned time phase assigned to the device adapter, to allow a non-realtime device to transmit data during the assigned time phase assigned tothe device adapter; and determining whether to defer transmission ofdata during the free access phase if the device adapter has data totransmit during the free access phase, to allow a non-real time deviceto transmit data during the free access phase.
 10. An apparatus,comprising: means for obtaining a substantially synchronized time framewhich is maintained among a plurality of device adapters interconnectedby a network, the time frame repeating periodically and including aplurality of assigned time phases and a free access phase; means forcontrolling transmission of data during at least one of an assigned timephase, assigned to the apparatus prior to transmission of real-time databy the apparatus and included in the plurality of assigned time phases,or the free access phase; means for refraining from transmitting dataduring time phases not assigned to the apparatus; and means fordetermining whether to defer transmission of data during the assignedtime phase assigned to the device adapter, if the device adapter hasdata to transmit during the assigned time phase assigned to the deviceadapter, to allow a non-real time device to transmit data during thetime phase assigned to the device adapter; and means for determiningwhether to defer transmission of data during the free access phase ifthe device adapter has data to transmit during the free access phase, toallow a non-real time device to transmit data during the free accessphase.
 11. A master timing device, comprising: a transmission unitconfigured to transmit a synchronization signal at regular intervals tosynchronize local clocks of each of a plurality of device adapters; amaintaining unit configured to maintain a substantially synchronizedtime frame among the plurality of device adapters interconnected by anetwork, the time frame repeating periodically and including a pluralityof assigned time phases and a free access phase; and an assigning unitconfigured to assign each time phase to a specific device adapter,wherein a respective one of the plurality of device adapters isconfigured to transmit data during at least one of an assigned timephase assigned to the respective one of the plurality of device adaptersprior to transmission of real-time data by the respective one of theplurality of device adapters or the free access phase, to refrain fromtransmitting data during time phases not assigned to the respective oneof the plurality of device adapters, and to determine whether to defertransmission of data during at least one of the assigned time phase orthe free access phase to allow a non-real time device to transmit data.12. The master timing device of claim 11, wherein the transmission unitis configured to send the synchronization signal every predeterminednumber of frames.
 13. The master timing device of claim 11, wherein themaster timing device is one of a plurality of device adapters.
 14. Amethod, comprising: transmitting a synchronization signal at regularintervals to synchronize local clocks of each of a plurality of deviceadapters; maintaining a substantially synchronized time frame among theplurality of device adapters interconnected by a network, the time framerepeating periodically and including a plurality of assigned time phasesand a free access phase; and assigning each time phase to a specificdevice adapter prior to transmission of real-time data by the specificdevice adapter, wherein a respective one of the plurality of device isconfigured to transmit data during at least one of a respective assignedtime phase or free access phase, to refrain from transmitting dataduring time phases not assigned to the respective one of the pluralityof device adapters, and to determine whether to defer transmission ofdata during at least one of the assigned time phase or the free accessphase to allow a non-real time device to transmit data.
 15. Anapparatus, comprising: means for transmitting a synchronization signalat regular intervals to synchronize local clocks of each of a pluralityof device adapters; means for maintaining a substantially synchronizedtimeframe among the plurality of device adapters interconnected by anetwork, the time frame repeating periodically and including a pluralityof assigned time phases and a free access phase; and means for assigningeach time phase to a specific device adapter prior to transmission ofreal-time data by the specific device adapter, wherein a respective oneof the plurality of device adapters is configured to transmit dataduring at least one of a respective assigned time phase or the freeaccess phase, to refrain from transmitting data during time phases notassigned to the respective one of the plurality of device adapters, andto determine whether to defer transmission of data during at least oneof the assigned time phase, or the free access phase to allow a non-realtime device to transmit data.
 16. The device adapter of claim 1, whereinthe lengths of at least two of the assigned time phases or free accessphase vary from one frame to another frame, while the frame lengthsremain constant.
 17. The device adapter of claim 1, wherein the deviceadapter is further configured to determine whether or not to defertransmission of data based at least in part on a deadline fortransmitting the data.
 18. The method of claim 5, wherein the lengths ofat least two of the assigned time phases or free access phase vary fromone frame to another frame, while the frame lengths remain constant. 19.The method of claim 5, wherein at least one of said determining whetherto defer transmission of data during the assigned time phase assigned tothe device adapter to said determining Whether to defer transmission ofdata during the free access phase includes determining whether or not todefer transmission of data based at least in part on a deadline fortransmitting the data.
 20. The medium of claim 9, wherein the lengths ofat least two of the assigned time phases or free, access phase vary fromone frame to another frame, while the frame lengths remain constant. 21.The medium of claim 9, wherein at least one of said determining whetherto defer transmission of data during the assigned time phase assigned tothe device adapter or said determining whether to defer transmission ofdata during the free access phase includes determining whether or not todefer transmission of data based at least in part on a deadline fortransmitting the data.
 22. The master timing device of claim 11, whereinthe lengths of at least two of the assigned time phases or free accessphase vary from one frame to another frame, while the frame lengthsremain constant.
 23. The master timing device of claim 11, wherein atleast one of the plurality of device adapters is further configured todetermine whether to defer transmission of data during at least one ofthe assigned time phase or the free access phase based at least in parton a deadline for transmitting the data.
 24. The method of claim 14,wherein the lengths of at least two of the assigned time or free accessphase vary from one frame to another frame, while the frame lengthremain constant.
 25. The method of claim 14, wherein at least of theplurality of device adapters is further configured to determine weatherto defer transmission of data during at least one of the assigned timephase or the free access phase based at least in part on a deadline fortransmitting the data.